1. Technical Field
This invention relates to a polarizing lens and method of its manufacture.
2. Background Art
With the recent increase in popularity of outdoor activities such as outdoor sports and recreation, sunglasses have been increasingly used worldwide to protect the eyes from sunlight. Sunglasses have polarizing lenses, which are roughly classified into those of plastic and glass. Generally, plastic polarizing lenses tend to be more commonly used because they are lighter in weight, easier to handle, and higher in safety than glass polarizing lenses.
Plastic polarizing lenses employ such a structure that a polarizing film consisting of polyvinyl alcohol or the like is interposed between plastic resins. Typical examples include polarizing lenses produced from CR-39 by cast type molding.
However, polarizing lenses consisting of CR-39 are problematic in that they are not sufficiently impact-resistant and thus are subject to cracking. Additionally, the cast type molding, although a suitable production method of thermosetting resins such as CR-39, requires a long period of time to polymerize and cure the resin inside the mold, which necessitates a large number of molds of a single standard. This posed the problem of poor productivity.
Meanwhile, such polarizing lenses are known that polycarbonate is laminated by injection molding on the rear side of a polarizing film (e.g., Japanese patent publication Hei08-52817). These polarizing lenses have increased strength and superior impact resistance, and additionally, compared with cast type molding, the takt time (cycle time) is significantly shorter and they can be made with only a small number of molds to prepare; thus productivity can be improved.
However, it has become evident that polarizing lenses consisting of injection-molded polycarbonate are subject to iridescent patterns such as color shading and distortion, falling short of the optical properties required of polarizing lenses. Inherently, polarizing lenses allow passing of light waves oscillating in particular directions through the lenses on the basis of the properties of the polarizing films, but the light waves are made to rotate or bend by the birefringence of the polycarbonate, which is believed to be a main factor of iridescent patterns such as color shading and distortion.
Polycarbonate has a high stress-optic coefficient and thus birefringence easily occurs upon application of a small level of stress. It is therefore inevitable that birefringence is formed in injection-molded polycarbonate due to fluid residual stress or thermal stress involved in the injection filling and cooling steps.
However, the iridescent patterns such as color shading and distortion that occur on the polarizing lenses are not preferred because they can adversely affect the eyesight and the field of vision.